Sensitive electronics are often prone to radio frequency interference (RFI) as well as electromagnetic interference (EMI) at frequencies ranging up to several megahertz. The electrical performance of an analog or digital circuit can be impaired significantly by such types of interference. In addition, vibrational energy impinging upon an enclosure containing such circuits and transmitted into the electronic components themselves can impair circuit performance to a degree dependent upon the microphonic sensitivity of the electronic components and the internal wiring used. Sources of the RFI, EMI, and vibrational energy are assumed to be external to the enclosure and may include other high frequency electronics (especially digital circuitry), electrical motors, transformers, AC power lines (and cables), electrical switches, electrical lighting, loudspeaker output, etc. In addition to preventing RF, magnetic and vibrational energy from entering the circuitry contained within the enclosure, it is desirable to prevent any RF or magnetic energy generated by the contained circuitry from leaving the enclosure, where it could affect other sensitive electrical circuits.
Electronics are usually housed within an enclosure formed of either aluminum, steel, or plastic. Plastic enclosures offer no protection from RFI or EMI unless they are sprayed with a highly conductive coating (usually a metal) and properly grounded. This can provide a degree of RFI protection, but offers very little protection from magnetic interference, except at very high frequencies (&gt;100 kHz). Steel enclosures offer some protection from magnetically generated interference, but sacrifice a significant degree of RFI protection due to the fairly poor electrical conductivity of steel at very high frequencies. This can be remedied somewhat by plating the steel with a very conductive metal such as copper. However, steel is still not entirely effective in preventing low frequency (e.g., 60 Hz) magnetic interference, especially if the field strength is high, because of the marginal magnetic permeability of the steel materials typically used. An aluminum enclosure offers good protection from RFI and very high frequency EMI (assuming correct grounding guidelines are followed) due to its high degree of electrical conductivity. Aluminum enclosures, however, offer very little protection from EMI at frequencies below 100 kHz since they are not magnetically permeable, i.e., they are transparent to low frequency magnetic fields. It is apparent that none of the commonly used materials for electronics enclosures offers a high degree of both RFI and EMI protection, particularly EMI protection at low frequencies. In addition, immunity from vibrational energy is rarely addressed correctly in most enclosure designs. Often, materials with dubious damping qualities are used. In most cases, even when the appropriate materials are utilized, the surface area coverage is inadequate to provide any significant amount of vibrational energy damping, particularly at lower frequencies, below 200 Hz. It should be emphasized that usually, immunity from vibrational energy is not addressed at all.
It is therefore an object of the present invention to prevent RF, magnetic and vibrational energy from entering the circuitry contained within an enclosure. It is a further object of the invention to provide an enclosure that prevents entry of such energy, while preventing any RF or magnetic energy generated by the contained circuitry from leaving the enclosure, where it could affect other sensitive electrical circuits. It is yet another object of the invention to provide an enclosure that does not have the detrimental qualities of enclosures formed solely of either aluminum, steel, or plastic.